Because mercury having high toxicity is included in flue gas discharged from a boiler, which is a combustion device in a thermal power plant or the like, conventionally, various systems for removing mercury in the flue gas have been studied.
Normally, a wet type desulfurizer for removing sulfur contents in flue gas is provided in boilers. In flue-gas treating facilities where a desulfurizer is attached to a boiler as flue-gas treating device, it is widely known that when chlorine contents (Cl) in the flue gas increase, the percentage of divalent metallic mercury soluble in water increases, and thus the desulfurizer can easily collect mercury.
Therefore, recently, various methods and apparatuses for treating metallic mercury have been devised by combining NOx removing apparatus that reduces NOx and a wet type desulfurizer that uses an alkaline absorbent as an SOx absorbent (Patent Document 1).
As a method for treating metallic mercury in flue gas, a removal method using an adsorbent such as activated carbon or a selenium filter has been known. However, this method requires a special adsorption removal unit, and thus it is not suitable for treatment of large-capacity flue gas such as flue gas from a power plant (Patent Document 2).
FIG. 7 is a schematic configuration diagram of a flue gas control system of a coal combustion boiler.
As shown in FIG. 7, a conventional flue gas control system includes NOx removing apparatus 13 that removes nitrogen oxides in flue gas from a coal combustion boiler 11 by addition of ammonia 12, an air preheater 14 that recovers heat in the gas after removal of nitrogen oxides, a precipitator 15 that removes particulates in the gas after heat recovery, a gas-liquid contact type desulfurizer 16 that removes sulfur oxides in the gas after particulate collection by a limestone gypsum method and also removes mercury oxides, and a stack 17 that discharges purged gas which has undergone desulfurization and mercury removal to outside. Further, in the conventional flue gas control system, a bag filter 21 is installed on a downstream side of the precipitator 15, to supply activated carbon 22a from an activated carbon atomizer 22 into a gas passage, so that mercury in the gas is adsorbed and removed. In FIG. 7, reference numeral 18 denotes air. By feeding the air 18, an oxidation reduction potential in the desulfurizer 16 is adjusted.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-7612
Patent Document 2: Japanese Patent Application Laid-open No. 2005-230810